All-optical timing restoration

ABSTRACT

Timing restoration for a series of input pulses is performed optically in a transmission or switching system, using a nonlinear material with negligible walk-off that also receives an essentially orthogonally polarized series of reference pulses. In the nonlinear material, the input pulses are frequency shifted by the presence of the reference pulses. For a material with negligible walk-off, the frequency shift only occurs when the pulses partially overlap, but not when the pulses are coincident. The frequency shifted output from the nonlinear material is supplied to a dispersive delay line that translates the frequency shift into a time shift, such that the input pulses are retimed by the reference pulses. If the nonlinear material has a nonlinear index of refraction n 2  &gt;0, then the dispersive delay line must have an anomalous Group Velocity Dispersion (GVD); on the other hand, if the nonlinear material has an index of refraction n 2  &lt;0, then the dispersive delay line must have a normal GVD. The nonlinear and dispersive media may be separate sections or distributed in the same medium.

BACKGROUND OF THE INVENTION

In optical switching and transmission systems, it is important toperiodically restore the logic level and timing of pulses traveling inthe optical transmission medium. Such restoration is currently performedin regenerators, which typically include electro-optical devices. Thecurrent trend toward all optical systems has resulted in development oferbium-doped fiber amplifiers, which when used with soliton pulses,correct the pulse amplitude and shape and thus provide logic levelrestoration without the need for optical to electrical conversion. Whensuch amplifiers are used, timing restoration is still needed sincewithout such restoration the transmission or switching system can becomelimited by timing jitter and fluctuations (e.g. from backgroundspontaneous emission, temperature variations, etc.). To date, an opticaldevice for performing such restoration has not been available. However,other advances in soliton transmission and switching systems, such asthe ultra-fast optical logic devices, described in U.S. Pat. No.4,932,739 issued to applicant on Jun. 12, 1990, and all-optical timedomain chirp switch described in U.S. Pat. No. 5,078,464 issued toapplicant on Jan. 7, 1992, and assigned to the same assignee as thepresent application, are available for use in helping to address theproblem described above.

SUMMARY OF THE INVENTION

In accordance with the present invention, timing restoration for asequence or series of input pulses is performed optically in atransmission or switching system, using a nonlinear material withnegligible walk-off that also receives an essentially orthogonallypolarized sequence or series of reference pulses. In the nonlinearmaterial, the input pulses are frequency shifted by the presence of thereference pulses. For a material with negligible walk-off, the frequencyshift only occurs when the pulses partially overlap, but not when thepulses are coincident. The frequency shifted output from the nonlinearmaterial is supplied to a dispersive delay line that translates thefrequency shift into a time shift, such that the input pulses areretimed by the reference pulses. If the nonlinear material has anonlinear index of refraction n₂ >0, then the dispersive delay line musthave an anomalous Group Velocity Dispersion (GVD); on the other hand, ifthe nonlinear material has an index of refraction n₂ <0, then thedispersive delay line must have a normal GVD. The nonlinear anddispersive media may be separate sections or may be distributed in thesame medium.

In one embodiment, the invention can be realized by a hybrid time domainchirp switch (TDCS) similar to the one described in our above-citedpatent application, that consists of a nonlinear chirper (which may beimplemented in a semiconductor waveguide) followed by a dispersive delayline (which may be implemented in a polarization maintaining fiber). Thechirp switch described in the above-cited patent application ismodified, however, by arranging the nonlinear chirper (semiconductorwaveguide) so that it exhibits negligible walk-off.

Another all-fiber embodiment of the nonlinear chirper may be apolarization holding fiber in the anomalous GVD regime that is made tolook "non-birefringent". This can be accomplished by taking severalsegments of polarization maintaining fiber, exchanging the fast and slowaxes alternately, and then splicing the segments together. To avoidwalk-off, each segment should be less than half a walk-off length.

Using a timing restorer in accordance with our invention, an all-opticalregenerator for solitons can be achieved by cascading our invention(e.g., a hybrid TDCS) with an optical amplifier such as an erbium-dopedfiber amplifier. The regenerator advantageously will thus providerestoration of the amplitude, pulse shape, and timing.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood by reference to thefollowing Detailed Description, which should be read in light of theaccompanying drawings in which:

FIG. 1 is a schematic of a Time Domain Chirp Switch (TDCS), the detailsof which are described in the above-cited U.S. Pat. No. 5,078,464;

FIG. 2 illustrates one implementation of the TDCS of FIG. 1;

FIG. 3 illustrates in block diagram form, the elements of the presentinvention;

FIG. 4 illustrates one embodiment of the present invention that utilizesa hybrid TDCS comprising a semiconductor waveguide and a polarizationmaintaining fiber;

FIG. 5 illustrates time shift keyed data for the hybrid TDCS of FIG. 4,with a switching energy of 9.8 pJ and a fan-out of three;

FIG. 6 illustrates shift of the control pulse versus the signal energyfor a control energy of 96.5 pJ in the waveguide of FIG. 4;

FIG. 7 illustrates the shift of the control pulse versus the initialseparation between the control and signal pulses for the hybrid TDCS ofFIG. 4;

FIGS. 8 and 9 illustrate the temporal correcting nature of the nonlinearinteraction for n₂ >0 and anomalous group velocity dispersion in theembodiment of FIG. 4;

FIG. 10 illustrates for the embodiment of FIG. 4 the calculatedfrequency shift of an input pulse because of cross-phase-modulation, inwhich β=1 corresponds to equal widths of the input and reference pulse,while β=0.5 corresponds to a reference pulse twice as wide as the input;

FIG. 11 illustrates another, all-fiber embodiment of a nonlinear chirperwhich can be used in a timing restorer in accordance with the presentinvention; and

FIG. 12 illustrates in block diagram form the use of a timing restorersuch as the restorer of FIGS. 3, 4 or 11, in an all-optical regenerator.

DETAILED DESCRIPTION

The above-cited copending application describes a time domain chirpswitch (TDCS) logic gate shown in FIG. 1 that is based on solitondragging in optical fibers. The switch, which has been implemented withswitching energies approaching one picojoule, consists of a nonlinearchirper 101 followed by a soliton dispersive delay line 102. The digitallogic that can be implemented by the switch of FIG. 1 is based on timeshift keying in which a logical one corresponds to a control pulseapplied on input 103 that arrives within a clock window as compared toan essentially orthogonally polarized signal pulse that is applied oninput 104, and a logical zero corresponds to a control pulse thatarrives outside of the clock window. One embodiment of the TDCS,illustrated in FIG. 2, includes a moderately birefringent fiber 201 thatcorresponds to nonlinear chirper 101 of FIG. 1, and a polarizationmaintaining fiber 202 coupled to a polarizer 205 that corresponds tosoliton dispersive delay line 102 of FIG. 1. The control pulses 203 andsignal pulses 204 applied to the TDCS are orthogonally polarized toavoid linear interference between the pulses. Only control pulses arepassed through polarizer 205 and appear at control output 103 in FIG. 1.

In accordance with the present invention, the TDCS of FIG. 1 is modifiedto yield a "hybrid" TDCS as shown in FIG. 3 by replacing nonlinearchirper 101 with a nonlinear chirper 301 having negligible walk-off. Bythis, we mean that the orthogonally polarized control and signal pulseshave the same velocity, and, therefore, maintain their relativepositions while propagating through the nonlinear material. In thenonlinear chirper, the control or input pulse is frequency shifted (or"chirped") by the presence of the reference or signal pulse. For amaterial with negligible walk-off, the frequency shift only occurs whenthe pulses partially overlap, but not when the pulses are coincident.The output of chirper 301 is coupled to dispersive delay line 302, thatexhibits anomalous GVD if n₂ >0, and that exhibits normal GVD if n₂ <0.The GVD in dispersive delay line 302 translates the frequency shiftprovided by chirper 301 into a time shift. By arranging for thepreviously mentioned n₂ and GVD combinations, the desired timingrestoration or correction is obtained. Dispersive delay line 302 may bea fiber or bulk optical elements such as gratings or prisms, providedthat in all events, this component is arranged such that the pulse shapeand width are not appreciably distorted. For example, if dispersivedelay line 302 is implemented as a long length of optical fiber, it maybe desirable to use soliton pulses rather than linear pulses.

FIG. 4 illustrates one embodiment of the hybrid TDCS timing restorer ofFIG. 3 in which nonlinear chirper with negligible walk-off 301 of FIG. 3is implemented by an AlGaAs semiconductor waveguide 401 (described inmore detail below) and in which soliton dispersive delay line 302 (as inFIG. 2) is implemented by polarization maintaining fiber 402 coupled toa polarizer 403 that only passes control pulses. The GVD of fiber 402 isarranged as indicated in FIG. 3.

The hybrid TDCS shown in FIG. 4 was tested in experimental apparatusthat included a passively modelocked color center laser that suppliesτ˜415 fsec pulses near 1.69 μm. Delay line 302 was implemented as 600 mof polarization maintaining, dispersion-shifted fiber 402 with a zerodispersion wavelength of 1.585 μm (group velocity dispersion at 1.69 μmis about 6 ps/(nm-km)). Waveguide 401 was 2.1 mm long and had across-sectional area of approximately 2.5 μm×5 μm. It was formed as aridge waveguide in a 2.55 μm thick layer of Al₀.2 Ga₀.8 As; guiding wasassured by a 2.55 μm buffer layer of Al₀.5 Ga₀.5 As that had arefractive index 0.15 less than the active layer. A large waveguide waschosen for ease of coupling using bulk optics, and, although severalspatial modes were supported, the fiber afterwards acted as a spatialfilter to favor the lowest order mode. The semiconductor materialcomposition was chosen so that the laser spectrum lies more than 100 meVbelow the half-gap energy, thus avoiding two photon absorption. In thiswavelength range it was found that n₂ ˜3×10⁻¹⁴ cm² /W and that thematerial is isotropic (e.g. cross-phase modulation is two-thirds ofself-phase modulation). In this experiment a π-phase shift fromself-phase modulation with less than a ten percent absorption wasobtained, and it was found that the nonlinear absorption originatedprimarily from three photon absorption. Furthermore, time resolvedpump-probe measurements confirmed that the nonlinearity wasinstantaneous on the 500 fs time scale of the pulses.

The time shift keyed data for the hybrid TDCS of FIG. 4 is illustratedin FIG. 5, where the signal energy in the waveguide is 9.8 pJ and thecontrol energy is 96.5 pJ. Rectangle 501 outlines the clock window, andit is noted that adding the signal shifts the control pulse out of thiswindow. Because of mode mismatch and poor coupling into the fiber, thecontrol energy exiting the fiber is 30.2 pJ, yielding a device fan-outor gain of about 3. Nonlinear phase shifts based on earlier nonlinearspectroscopy in longer lengths of the same waveguide were estimated. Thepeak self-phase-modulation phase shift for the control pulse in thewaveguide was found to be about π/3, while the peakcross-phase-modulation phase shift imposed on the control by the signalwas about π/40. The measured shift of the control pulse as a function ofsignal energy is illustrated in FIG. 6, and to the lowest order, it isexpected that the shift will be linearly proportional to the switchingenergy.

It has been shown by M. N. Islam, C. R. Menyuk, C. J. Chen and C. E.Soccolich, Opt. Lett. 16, 214 (1991), that in soliton dragging, thecontrol pulse chirps because of the combined action of cross-phasemodulation and walk-off: the walk-off asymmetrizes the frequencymodulation, thereby leading to a shift in the center frequency of thepulse. However, time-resolved measurements in waveguides as long as 7.7mm indicate negligible walk-off for the 415 fs pulses used in theexperiments. To understand how the present invention achieves time shiftwithout birefringence, (i.e., using a semiconductive waveguide 401 asshown in FIG. 4) the shift of the control pulse ΔT as a function of theseparation between the control and signal pulses δt, was studiedexperimentally, as shown in FIG. 7. Note in that figure that the resultsshowed that there is not time shift when the two pulses are prefectlyoverlapped. On the other hand, when the pulses are partially separatedand one pulse travels on the wing or side of the other pulse, there canbe a net chirp or shift of the center frequency. The finite duration ofthe shift in FIG. 7 confirms that the nonlinearity is instantaneous andfollows the pulse envelope.

The data in FIG. 7 also shows that the hybrid TDCS can provide thetiming restoration that is desired. For example, suppose that thecontrol pulse is the input pulse and that the signal pulse is a"reference" pulse with the proper temporal position. We define theseparation between the pulses δt=t_(input) -t_(ref) and the shift of theinput pulse ΔT. Therefore, if the input pulse is earlier than thereference pulse (δt<0), then the nonlinear interaction pulls the inputpulse to later times (ΔT>0), and vice versa. To precisely correct fortiming errors, the reference pulse level can be adjusted so the slope ofthe timing curve near δt˜0 is unity.

An intuitive picture of the timing correction results from consideringthe frequency shift and anomalous group velocity dispersion. Theinstantaneous frequency change δω of the input pulse is given by R. H.Stolen and Chinlon Lin, Phys. Rev. A. 17, 1448 (1978), as follows:##EQU1## and is proportional to the negative slope of the referencepulse for n₂ >0. Higher frequencies travel faster in anomalous GVDmaterial, or a pulse with increased frequency arrives earlier andtravels toward the left on a time axis.

FIGS. 8 and 9 illustrate the correcting nature of the interaction. Whenthe input pulse 801 arrives earlier than the reference pulse 802, inputpulse 801 sees a positive slope, which lowers its instantaneousfrequency, which in turn slows pulse 801 and moves it to a later time.When the input pulse 902 arrives later than the reference pulse 901,input pulse 902 sees a negative slope, which raises its instantaneousfrequency, which in turn speeds up pulse 902 and moves it to an earliertime. Note that proper operation requires n₂ >0 and anomalous dispersionin the fiber, or vice versa signs, and this simple picture holds as longas there is negligible walk-off between the two pulses. The precedingdescription is general, and is not limited to specific pulses; solitonsmay be desirable, but are not necessary.

Simple formulas describe the shift of the input pulse center frequencyΔω_(c) due to cross-phase-modulation in the limit of negligibledispersion, (no dispersion means that the pulses do not change in shapeduring the interaction). Standard soliton normalizations described byMollenauer et al. in IEEE J. Quantum Electron, QE-22, 157 (1986) areused in the following equations, and t is local time on the pulse whilez is distance along the waveguide. The case with negligible walk-off isdescribed below, while the case of moderate birefringence is treated inthe above-cited article in Optics Letters, Volume 16.

In the following analysis it is assumed that the input u and reference vpulses are given by

    u(z=0,t)=sech(t);|v(z,t)|.sup.2 =A.sub.s.sup.2 sech.sup.2 (β(t+δt))                                      (2)

where δt=t_(input) -t_(ref) and β=τ_(input) /τ_(ref). The input pulseaccumulates a nonlinear phase shift due to cross-phase-modulation and isproportional to ##EQU2## Ther is also a phase shift arising fromself-phase-modulation, but for a symmetric pulse this only leads to asymmetric broadening of the spectrum without a shift in the centerfrequency.

The shift in the frequency centroid is given by ##EQU3## and note thatΔω_(c) =0 when A_(s) =0. Introducing Eqs. (3) into (4) we obtain It canbe checked from symmetry arguments that G(δt=0)=0, as expected in theabsence of walk-off. In FIG. 10, a plot illustrates the negative of G(β,δt), which is proportional for anomalous GVD to ΔT/τ of the input pulse,for different values of β, and it is found that there is qualitiveagreement with the experimental data of FIG. 7. The experimental datamay drop off more abruptly because the laser pulses are Gaussian ratherthan hyperbolic secant. The asymmetry in the experimental data maypartially be due to slightly asymmetric pulses from the laser.

The timing restoration provided by the hybrid TDCS of FIG. 4 can betailored by adjusting the characteristics of the reference pulse. Thewidth of the timing window can be adjusted by changing the width of thereference pulse, and the slope of the correction can be adjusted bychanging the intensity of the reference pulse. For example, as shown inFIG. 10, by doubling the pulse width of the reference, the timingcorrection window can be increased from 1.8τ (β=1) to 2.4τ (β=0.5).

Although experiments were carried out with 415 fs pulses, the presentinvention can be extended to different pulse widths as well as to othernonlinear materials with negligible walk-off. For example, as shown inFIG. 11, a "non-birefringent" fiber that can maintain polarizationextinction could be used as nonlinear chirper 301 of FIG. 3. as thehybrid TDCS of FIG. 4. In this embodiment a number of optical fibersegments 1101, 1102, 1103 each having its slow and fast axes crossed,can be spliced together. In this arrangement, the length of each segmentshould be less than half of the walk-off length. Since optical fibershave n₂ >0, the device should be operated in the anomalous GVD regime.

The use of a timing restorer in accordance with the present invention inan all optical pulse regenerator, is illustrated in FIG. 12. As shown, astream or series of soliton pulses on input 1200 are applied to anoptical amplifier 1201, which may be anerbium-doped fiber amplifier, inorder to restore the amplitude and shape of the pulses. Thereafter, thepulses are applied to a timing restorer 1202, which receives a stream orseries of reference pulses on input 1203. Restorer 1202, which may beimplemented as shown generally in FIG. 3, and specifically as shown inFIGS. 4 or 11, restores the input pulses on line 1200 to the timingprovided by the reference pulses. The regenerator of FIG. 12 is alloptical, and does not require elecro-optical components.

Numerous modifications and improvements can be made to the presentinvention, as will be understood by those skilled in the art. Forexample, by increasing the nonlinearity n₂, less reference pulse energyis required, or by modifying the temporal profile of the referencepulse, one can tailor the shape of the time window. Accordingly, it isto be understood that the invention is to be limited only by theappended claims.

I claim:
 1. Apparatus in an optical communications system for restoringthe timing in a series of input pulses in accordance with a series ofreference pulses, said apparatus comprisinga nonlinear material havingnegligible walk-off and having a nonlinear index of refraction n₂ forreceiving said input pulses with a first polarization orientation andsaid reference pulses with a second polarization orientation essentiallyorthogonal to said first polarization orientation; and a dispersivedelay line for receiving pulses output from said nonlinear material,said delay line having anomalous Group Velocity Dispersion (GVD) if n₂is greater than 0 and normal GVD if n₂ is less than
 0. 2. The inventiondefined in claim 1 wherein said non-linear material comprisesa pluralityof connected segments of polarization maintaining fiber having anonlinear index of refraction n₂, each of said segments having a fastaxis and a slow axis, said segments being connected such that said fastand slow axes of alternate segments are exchanged.
 3. The inventiondefined in claim 2 wherein the length of each of said segments is lessthan half a walk-off length.
 4. Apparatus for retiming a sequence ofinput pulses in an optical transmission medium in accordance with areference sequence of pulses, comprisingmeans for shifting the frequencyof pulses in said input sequence of pulses due to the presence of saidreference sequence of pulses, and means for advancing or retarding thepulses in said input sequence as a function of said pulse frequency. 5.The invention defined in claim 4, wherein said frequency shifting meansincludes a nonlinear material with negligible walk-off, andwherein saidadvancing or retarding means includes a dispersive delay line.
 6. Theinvention defined in claim 5, wherein said nonlinear material has anonlinear index of refraction of n₂ and wherein said dispersive delayline has anomalous Group Velocity Dispersion (GVD) if n₂ is positive andnormal GVD if n₂ is negative.
 7. Apparatus in an optical communicationssystem for altering the timing in a series of input pulses in accordancewith a series of essentially orthogonally polarized reference pulses,includinga nonlinear material receiving said input pulses and saidseries of essentially orthogonally polarized reference pulses, saidnonlinear material arranged to shift the frequency of said input pulsesas a result of the presence of said reference pulses in said material,and a dispersive delay line receiving said input and reference pulses,said delay line arranged to translate said frequency shift produced bysaid nonlinear material into a corresponding time shift.
 8. Theinvention defined in claim 7 wherein said input and reference pulses aresoliton pulses.
 9. The invention defined in claim 8 wherein saidnonlinear material is a semiconductor waveguide.
 10. The inventiondefined in claim 9 wherein said semiconductor is comprised of AlGaAs.11. The invention defined in claim 8 wherein said dispersive delay lineis a polarization maintaining optical fiber.
 12. In an opticalcommunications system, a method of restoring the timing in a series ofinput pulses with a first polarization orientation in accordance with aseries of reference pulses with a second polarization orientationorthogonal to said first polarization, said method comprising the stepsofapplying said input pulses and said reference pulses to a nonlinearmaterial having negligible walk-off and having a nonlinear index ofrefraction n₂ ; and applying pulses output from said nonlinear materialto a delay line having anomalous Group Velocity Dispersion (DVD) is n₂is greater than 0 and normal GVD if n₂ is less than
 0. 13. The method ofclaim 12 wherein said non-linear material includesa plurality ofconnected segments of polarization maintaining fiber having a nonlinearindex of refraction n₂, each of said segments having a fast axis and aslow axis, said segments being connected such that fast and slow axes ofalternate segments are exchanged.
 14. The invention defined in claim 13wherein the length of each of said segments is less than half a walk-offlength.
 15. A method of retiming a sequence of input pulses in anoptical transmission medium in accordance with a reference sequence ofpulses, comprising the steps ofshifting the frequency of pulses in saidinput sequence of pulses due to the presence of said reference sequenceof pulses, and advancing or retarding of pulses in said input sequenceas a function of said pulse frequency.
 16. The method defined in claim15, wherein said frequency shifting step includes applying said pulsesto a nonlinear material with negligible walk-off, andwherein saidadvancing or retarding step includes applying said pulses to adispersive delay line.
 17. The invention defined in claim 16, whereinsaid nonlinear material has a nonlinear index of refraction of n₂ andwherein said dispersive delay line has anomalous Group VelocityDispersion (GVD) if n₂ is positive and normal GVD if n₂ is negative. 18.In an optical communications system, a method of altering the timing ina sequence of input pulses in accordance with a series of essentiallyorthogonally polarized reference pulses, including the steps ofshiftingthe frequency of said input pulses as a result of the presence of asequence of essentially orthogonally polarized reference pulses, byapplying said input pulses and said reference pulses to a nonlinearmaterial, and translating said frequency shift produced by saidnonlinear material into a corresponding time shift by applying pulsesoutput from said nonlinear material to a dispersive delay line.
 19. Theinvention defined in claim 18 wherein said input and reference pulsesare soliton pulses.
 20. The invention defined in claim 18 wherein saidnonlinear material is a semiconductor waveguide.
 21. The inventiondefined in claim 20 wherein said semiconductor is comprised of AlGaAs.22. The invention defined in claim 18 wherein said dispersive delay lineis a polarization maintaining optical fiber.